I installed a MC-HD-AK4 (amp/harness/4 speakers) in my 2011 HD Roadlide w/factory radio and everything worked great. The distortion started after I installed a BTA6100 powered tube and harnessFAST-HD-SIK. If I unplug the fast harness from the amp it goes away. Also, the engine has to be running before it distorts. Anybody got a suggestion?

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Sounds like a ground issue. Verify your current ground connection with a DMM. Change the location if necessary.